1. Field of the Invention
The present invention is directed to a method to refine lubricant for a magnetic recording medium, a lubricant refined by the method, and a magnetic recording medium using the lubricant.
2. Description of the Related Art
Recording media such as fixed magnetic recording media are used as general-purpose data recording media in computers. A magnetic recording apparatus containing fixed magnetic recording media comprises a driving mechanism for the magnetic recording media, a driving mechanism for a magnetic head, a stopping mechanism for the magnetic head, and a data transfer mechanism, as well as one or more magnetic recording media. In recent years, the magnetic recording media have shown significant progress in increasing recording density, capacity, and data transfer rate.
A conventional fixed magnetic recording apparatus employs the contact start and stop (CSS) system, in which a magnetic recording head floats while the magnetic recording medium is rotating, and the magnetic head comes in contact with the surface of the magnetic recording medium when the motor for driving rotation of the magnetic recording medium stops. In this system, the magnetic head slides on the surface of the magnetic recording medium at the time of start and stop of rotation of the magnetic recording medium, and friction occurs between the magnetic recording medium and the magnetic head. To protect the magnetic layer from friction, a protective layer is provided, and a lubricant layer is provided to improve lubricity on the surface.
Recently, a rotating speed of a magnetic recording medium of a magnetic recording apparatus has increased from a traditional 5,400 rpm to a speed in a range of 7,200 to 15,000 rpm. As a result, a spin migration or spin off phenomenon, in which lubricant on the surface of a magnetic recording medium moves to a peripheral region or scatters from the medium surface, has become significant. Furthermore, the heat generated at high rotating speed evaporates the lubricant and introduces a problem of loss of the lubricant mass.
The thickness growth of the lubricant layer in the peripheral region of the magnetic recording medium induced by the spin migration causes adhesion obstruction. The evaporation of the lubricant may cause the protective layer to wear because of the decrease of the lubricant layer thickness. In the worst case, head crash occurs.
A lubricant layer of a magnetic recording medium needs to be in a stable form in a uniform thickness on a protective layer surface to avoid spin migration. In addition, strong adhesion and bonding are required at the interface between the lubricant layer and the protective layer. To enhance the adhesiveness, a perfluoropolyether lubricant having a terminal group of a hydroxy group or a piperonyl group is used. Such types of lubricant are available, for example, from Ausimont KK under the trade names of Fomblin Z DOL, Fomblin Z tetraol, and AM3001. Most of the presently used perfluoropolyether lubricant is prone to exhibit poor heat resistance in a very low molecular weight range, and high statical friction in a very high molecular weight range. Consequently, the lubricant now used has a number average molecular weight (Mn) in a range of 1,000 to 10,000, more specifically, a range of 2,000 to 4,000.
The above-described kinds of lubricant perform well in the range of the present rotating speed of a magnetic recording medium. However, at a higher rotating speed, a problem with durability against a phenomenon like spin migration may exist.
Japanese Unexamined Patent Application Publication No. 2001-229524 discloses the use of a lubricant that has a high molecular weight and a narrow distribution of molecular weight. Such a lubricant eliminated the problem accompanied by a conventional lubricant having a high molecular weight and increased adhesiveness with a protective layer, improving durability against spin migration.
While a lubricant having a high molecular weight was used in the Japanese Unexamined Patent Application Publication No. 2001-229524, a minimized flying height of a magnetic head raises a problem due to an effect of the lubricant molecules. Namely, the size of the lubricant molecules has an effect on the flight of the magnetic head, where there is a minimized distance between the magnetic head and the magnetic recording medium. Accordingly, if the durability against spin migration is similar, a lubricant with a lower molecular weight is preferable, as long as heat resistance is secured.
As described above, a lubricant needs improvement in view of the molecular weight, durability against spin migration, and heat resistance.
Besides the above problems, a commercially available lubricant contains impurities which need to be eliminated. For example, a commercially available perfluoropolyether lubricant contains impurities of perfluoropolyether having a terminal group such as —CF2Cl, —CF3, or —CF2H. These terminal groups exhibit a weaker polarity, as compared with a hydroxy group and a piperonyl group, which are the terminal groups intrinsic to a perfluoropolyether lubricant. If the content of the impurities is large, the adhesion and binding with a protective layer cannot be improved even if the molecular weight is large. Therefore, a refinement method to eliminate such impurities is required.
Japanese Unexamined Patent Application Publication No. 05-20673 discloses a method to refine a lubricant in which a technique is performed for refinement to control the molecular weight by using a gel permeation chromatography (GPC) method. Japanese Unexamined Patent Application Publication No. 11-199882 discloses a technique for refinement by controlling a molecular weight of a lubricant by using an organic solvent. A method to refine a lubricant using an organic solvent predominantly tends to eliminate highly polar and low molecular weight components. Consequently, the lubricant refined by the method may exhibit an increased proportion of the perfluoropolyether having a terminal group of —CF2Cl, —CF3, or —CF2H that shows a low solubility in an organic solvent. Japanese Unexamined Patent Application Publication No. 2001-164279 discloses a refinement method to increase the proportion of molecules having terminal groups that are intrinsic to a perfluoropolyether lubricant by applying supercritical fluid chromatography (SFC). While the reference states that the method is less expensive than a method using an organic solvent and achieves a higher productivity than a GPC method, problems may arise due to the cost and durability of a packing column in the SFC method when the mass treated in one process increases.
Further, a commercially available lubricant also includes ionic impurities as well as the above-described perfluoropolyether that has a weak polarity. The ionic impurities included in a lubricant existing on a recording medium causes corrosion, which induces dissolution of ions of metallic elements, such as cobalt, used in a magnetic layer. The cobalt ions catalyze decomposition of perfluoropolyether molecules used in the lubricant. Therefore, it is desirable to minimize the quantity of the ionic impurities.
Thus, the ionic impurities introduced into perfluoropolyether in its synthesis process need to be eliminated.